1. Field of the Invention
This invention relates generally to a hybrid fuel cell system and, more particularly, to a hybrid fuel cell system that employs a cell voltage controller, a damped driver power request module and a power balancing module for balancing the power between a battery and a fuel cell stack in the system to provide stack durability.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The dynamic power of a fuel cell system is limited. Further, the time delay from system start-up to driveability and low acceleration of the vehicle may not be acceptable. During a drive cycle, the stack cell voltage varies because the variable driver power request follows the stack polarization curve. The voltage cycles can decrease the stack durability. These drawbacks can be minimized by using a high voltage battery in parallel with the fuel cell stack. Algorithms are employed to provide the distribution of power from the battery and the fuel cell stack to meet the requested power.
For the reasons discussed above, some fuel cell vehicles are hybrid vehicles that employ a rechargeable supplemental power source in addition to the fuel cell stack, such as a DC battery or a super capacitor (also referred to as an ultra-capacitor or double layer capacitor). The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to a traction motor and other vehicle systems through a DC voltage bus line for vehicle operation. The battery provides the supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to meet the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery through the DC bus line.
FIG. 1 is a schematic block diagram of a fuel cell system 10 including a fuel cell stack 12 and a battery 14 that includes power electronics. In order to provide battery charge or discharge, a voltage difference is needed between the stack voltage and the battery voltage that is greater than or equal to the battery charge. When the stack voltage is greater than the battery voltage, the power electronics operates as a voltage amplifier where the gain is less than or equal to one. The fuel cell stack 12 provides electrical power to a high voltage bus line, represented here as positive bus line 16 and negative bus line 18. In a vehicle fuel cell system, the fuel cell stack 12 may include about 400 fuel cells. The battery 14 is also coupled to the high voltage bus lines 16 and 18, and provides supplemental power as discussed above.
The fuel cell system 10 includes a power inverter module (PIM) 22 electrically coupled to the bus lines 16 and 18 and an AC or DC traction motor 24. The PIM 22 converts the DC voltage on the bus lines to an AC voltage suitable for the AC traction motor 24. The traction motor 24 provides the traction power to operate the vehicle, as is well understood in the art. The traction motor 24 can be any suitable motor for the purposes described herein, such as an AC induction motor, an AC permanent magnet motor and an AC three-phase synchronous machine. During regenerative braking when the traction motor 24 is operating as a generator, electrical AC power from the motor 24 is converted to DC power by the PIM 22, which is then applied to the bus lines 16 and 18 to recharge the battery 14. A blocking diode (not shown) prevents the regenerative electrical energy applied to the bus lines 16 and 18 from flowing into the fuel cell stack 12, which could otherwise damage the stack 12.
It is known to maintain the output power of the stack within a desirable voltage range for as long as possible in order to increase fuel cell stack durability in a hybrid fuel cell system. For example, it is desirable to maintain a cell voltage for each fuel cell in the stack in the range of 0.725-0.85 volts. As the load on the fuel cell stack goes up, the cell voltage goes down, and vice versa. It is desirable to prevent each cell voltage from going above 0.85, which would be a very low stack load. Further, if the cell voltage falls below 0.725 volts for high loads, it is desirable to maintain the cell voltages in the high load range as long as possible for stack durability purposes. Also, it is desirable that the battery state of charge (SOC) does not go above its maximum charge limit or below its minimum charge limit.